Active flow management with hysteresis

ABSTRACT

The present invention provides for a computer network method and system that applies “hysteresis” to an active queue management algorithm. If a queue is at a level below a certain low threshold and a burst of packets arrives at a network node, then the probability of dropping the initial packets in the burst is recalculated, but the packets are not dropped. However, if the queue level crosses beyond a hysteresis threshold, then packets are discarded pursuant to a drop probability. 
     Also, according to the present invention, queue level may be decreased until it becomes less than the hysteresis threshold, with packets dropped per the drop probability until the queue level decreases to at least a low threshold. In one embodiment, an adaptive algorithm is also provided to adjust the transmit probability for each flow together with hysteresis to increase the packet transmit rates to absorb bursty traffic.

FIELD OF THE INVENTION

This invention relates in general to bandwidth allocation and activeflow management techniques for computer networks, and more particularlyto implementation of hysteresis in active flow management techniques toincrease network system throughput by making better use of availablequeue capacity.

BACKGROUND OF THE INVENTION

In computer network systems, active flow management techniques arecommonly used to control the subscription and offered load of eachthread data flow, along with the service rate of the network systemitself, to achieve fairness of bandwidth allocation.

However, unnecessary packet drop due to short burst traffic may occur,and problems arise if there is no mechanism provided to preferentiallytreat short bursts of packets. Most active queue management algorithmsdrop some packets when congestion is detected, and indeed in an initialburst to detect incipient congestion. If the burst is sustained for avery short period, this can cause unnecessary packet drops because thereis enough space in the packet buffer to be able to accommodate theburst. This is especially detrimental in Transmission Control Protocol(TCP) networks because each packet drop causes TCP retransmissions whichcan lead to very low useful throughput.

In TCP networks, it is known to use Explicit Congestion Notification(ECN) to mark packets and indicate to a sender that a congestion windowshould be adjusted to a lower rate. However, ECN applied in the case ofvery short and sustainable bursts can be detrimental to the totalthroughput because it unnecessarily causes the window to adjust when thepackets in the burst could well have been transmitted only with a littleprice in latency. It is also known to use an exponentially weightedmoving average of a queue level to smooth out bursts; however, thissolution is computationally expensive.

What is needed is a method and system for active flow management forcomputer networks that sustains short burst packet traffic withoutcausing unnecessary packet drops and at the same time not degrading thenetwork system throughput for persistent bursts of packets, and whichcan be implemented in hardware without too much logic overhead.

SUMMARY OF THE INVENTION

The present invention provides for a computer network method and systemthat applies “hysteresis” to an active queue management algorithm. If aqueue is at a level below a certain low threshold (L) and a burst ofpackets arrives at this network node, then the probability of droppingthe initial packets in the burst is recalculated, but the packets arenot dropped. However, if the queue level crosses beyond a “hysteresisthreshold” (Ht), then packets are discarded pursuant to a dropprobability. This allows more packets from the burst to get into thequeue. Where the burst lasts for a short time (a “short burst”), thenthe present invention provides the ability to transmit every singlepacket.

According to the present invention, when a queue level is beyond thehysteresis threshold, and arrival rate into the queue is less than thesending rate from the queue, then queue level is decreased until itbecomes less than the ‘hysteresis threshold’ (Ht). However, during thistime, packets get dropped as per the drop probability until the queuelevel decreases to at least the low threshold (L). Thus, the presentinvention is intended to improve network performance where a burst isreceived into a queue when the queue level is low.

In one embodiment of the present invention, an adaptive algorithm isalso provided to adjust the increment and decrement of transmitprobability for each flow, together with hysteresis to increase thepacket transmit rates by using packet data store to absorb burstytraffic. The proposed algorithm maintains the throughput for persistentbursts of packets. Using straight forward implementation of hysteresisin the active flow management will increase the system throughput bymaking better use of available queue capacity. This results in anincrease in the queue level peak, potentially exposing the system totail drops when subjected to severe bursts for bursty traffic. With theaddition of adaptive increment and decrement of transmit probability ofeach flow, queue peak can be limited to a reasonable level, thuspreventing tail drop.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation of the relationship of transmitprobability to buffer capacity according to the present invention.

FIG. 2 is a flowchart illustrating a hysteresis algorithm according tothe present invention.

FIG. 3 is a graphical representation of results for queue occupancy forhysteresis factor levels according to the present invention.

FIG. 4 is a graphical representation of the transmit rates for variouslevels of hysteresis according to the present invention.

FIG. 5 is a table of transmit rates for bursty traffic according to thepresent invention.

FIG. 6 is a graphical representation of a transmit rate throughputcomparison for bursty traffic for RED, RED with hysteresis, SARED andSARED with hysteresis according to the present invention.

FIG. 7 is a graphical representation of a latency comparison for burstytraffic for RED, RED with hysteresis, SARED and SARED with hysteresisaccording to the present invention.

FIG. 8 is a high-level block diagram of a standalone network simulatorappropriate for use with the present invention.

FIG. 9 is a graphical representation of UDP traffic on the standalonenetwork simulator of FIG. 9 according to the present invention.

FIG. 10 is a graphical representation of UDP short burst bursty trafficon the standalone network simulator of FIG. 9 according to the presentinvention.

FIG. 11 is a graphical representation of TCP constant traffic on thestandalone network simulator of FIG. 9 according to the presentinvention.

FIG. 12 is a graphical representation of TCP bursty traffic on thestandalone network simulator of FIG. 9 according to the presentinvention.

FIG. 13 provides tables of packet drop and packet transmit statisticsaccording to the present invention.

FIG. 14 is a flowchart that shows an adaptive algorithm according to thepresent invention.

FIG. 15 is a graphical representation of aggregate transmit ratesaccording to the present invention.

FIG. 16 is a graphical representation of average latency according tothe present invention.

FIG. 17 is a graphical representation of aggregate transmit ratesaccording to the present invention.

FIG. 18 is another graphical representation of aggregate transmit ratesaccording to the present invention.

FIG. 19 is a graphical representation of aggregate transmit rates forsteady traffic according to the present invention.

FIG. 20 illustrates an embodiment of the invention tangibly embodied ina computer program residing on a computer readable medium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention applies “hysteresis” to active queue managementtechniques. Hysteresis is generally defined as “the lagging of an effectbehind its cause” and it is well-known to use hysteresis behavior inapplications that switch between two transmission modes in a variety ofnetwork system applications. In the present invention, where a queue isat a level below a certain low threshold (L) and a burst of packetsarrives at this network node, then the probability of dropping theinitial packets in the burst is recalculated, but the packets are notdropped. However, if the queue level crosses beyond a “hysteresisthreshold” (Ht), then packets are discarded pursuant to a dropprobability. This allows more packets from the burst to get into thequeue. Thus, where the burst lasts for a short time (a “short burst”),then the present invention provides the ability to transmit every singlepacket.

According to the present invention, when a queue level is beyond thehysteresis threshold, and arrival rate into the queue is less than thesending rate from the queue, then queue level is decreased until itbecomes less than the ‘hysteresis threshold’ (Ht). If the arrival rateinto the queue is less than the sending rate from the queue, and packetsarriving into the queue are not discarded, the queue level willeventually rise and, after a while, no more packets will be acceptedinto the queue. According to algorithms taught by the present invention,arriving packets are randomly discarded but only to the point where thequeue level reaches the hysteresis threshold. However, during this timepackets get dropped as per the drop probability until the queue leveldecreases to at least the low threshold (L). Thus, the present inventionis intended to improve network performance where a burst is receivedinto a queue when the queue level is low.

Random Early Detection is a well-known queue management algorithm thatis used in network routers to detect incipient congestion based on queueoccupancy. As per the definition of RED, a low (L) and a high (H)threshold are defined for the queue. An exponentially weighted average(Qavg) of the queue occupancy is used to define congestion. If Qavg isless than L, the arriving packet is not discarded. If Qavg is greaterthan H, the arriving packet is discarded. If Qavg lies between L and H,a discard probability is calculated for the arriving packet based on thelinearly increasing probability between L and H.

One of the drawbacks of RED is that it is not easy to set the thresholdsto achieve optimum queue occupancy and network performance. Referringnow to FIG. 1, a graphical representation 10 of the relationship oftransmit probability p to buffer capacity Q according to the presentinvention is illustrated, where T represents the transmit probability;T₀ represents an initial value for the transmit probability; H and Lrepresent the High and low thresholds, respectively, for Random EarlyDetection (RED); and Qmax represents the buffer capacity. In the exampleillustrated, L=(¼)*Qmax; H=( −/32)*Qmax, 0<=L<=Ht<=Qmax;T=1−(Q−L)*(1−T₀)/(H−L), and T₀=⅛. However, the present invention is notlimited to these values, and it is to be understood that other valuesmay be selected for use with the present invention. The solid line 12 isthe transmit probability curve when the hysteresis flag is turned OFF.The dotted line 14 is the transmit probability curve when the hysteresisflag is turned ON. If hysteresis flag (HT) is on, then discards shouldbe delayed; otherwise, discard probability should be calculated as perthe queue management scheme. Thus, according to the present invention,the queue level is treated as a vector instead of a scalar: i.e., thequeue level now has a direction 16 along with a value 12 or 14.

FIG. 2 is a flowchart illustrating a hysteresis algorithm 301 forsetting the hysteresis flag according to the present invention. At step302, the hysteresis flag (HT) setting is ascertained. If the HT is on,then the queue level is checked at step 304 to determine whether it hasincreased beyond or is equal to the hysteresis threshold (Ht). If yes,then the HT flag is switched OFF at step 306. Otherwise, the HT flag iskept ON at step 308.

Alternatively, if the HT is OFF at step 302, then the queue level ischecked at step 310 to determine whether it has decreased to a valuebelow the low threshold (L): if not, then the HT flag remains switchedOFF at step 306; if so, then at step 312, it is determined whether theoffered load (OL) is below the link capacity (C). Offered load isdefined to be the aggregate traffic bandwidth being presented to thelink between two nodes in a network by a data transmitter. Link Capacityis defined as the maximum bandwidth supported by a physical link betweentwo nodes in a network. If OL is below C at step 312, then the HT flagis switched ON at step 3080N (this implies that the incoming flows arewell behaved); otherwise, the HT flag remains switched OFF at step 306.

The hysteresis algorithm 301 described above has been tested underconstant traffic load as well as under bursty traffic load. Embodimentsof the present invention have been implemented on two separate models: aNetwork processor simulation engine (NPSim) and an independent Networksimulator (ns-2). As shown in FIG. 3 et seq, there is considerableimprovement in network throughput by using hysteresis on RED accordingto the present invention. Under the constant load conditions, trafficsent into the NPSim network processor simulation model running thealgorithm 301 oversubscribed the link by 50%. A hysteresis factor (n) isdefined as the ratio of the hysteresis threshold to the low threshold ofRED, thus Ht=n*L. Results for queue occupancy for four levels of thehysteresis factor are shown in FIG. 3: level 402 where n=1; level 404where n=1.25; level 406 where n=1.5, and level 408 where n=2. Note thatapplying hysteresis to a constant load does not affect the throughput ofthe flow control scheme adversely. In fact, the initial hump 410 forcase 408 where n=2 helps to slightly improve the throughput.

FIG. 4 illustrates the transmit rates for various levels of Hysteresis.For case C1 502, the offered load (OL) is 50% of link capacity (C). Forcase C2 504, the offered load is 100% of link capacity. And for case C3506, the offered load is 150% of link capacity. Note that for C3 506,66.7% of traffic is transmitted.

FIG. 5 illustrates transmit rates for bursty traffic; 150% of linkcapacity is sent during the BURST ON period, and 20% of link capacity issent during the BURST OFF period. The BURST ON and OFF periods arevaried for different levels of oversubscription.

FIG. 6 illustrates a transmit rate throughput comparison for burstytraffic for RED 706, RED with hysteresis 702, Shock Absorber RandomEarly Detection (SARED) 708 and SARED with hysteresis 704. Furtherinformation about SARED is set forth in commonly-assigned U.S. patentapplication entitled “FLOW CONTROL IN COMPUTER NETWORKS”, Ser. No.10/160,507, filed Jun. 3, 2002, which is incorporated herein by thisreference. For Both RED and SARED queue management schemes, there is aconsiderable improvement in the transmit rates for bursty traffic whenhysteresis is applied to the scheme.

FIG. 7 illustrates a latency comparison for bursty traffic for RED 804,RED with hysteresis 802, SARED 810 and SARED with hysteresis 806. Thisillustration shows the price that needs to be paid in terms ofadditional queue occupancy (which can be translated into latency) forachieving the higher throughput. In all cases, the queue occupancyincreases only marginally.

FIG. 8 is a block diagram of a standalone network simulator NS-2 902 foruse with the present invention. A node s1 904 sends traffic to a node s2906 via router r1 910. An active queue management algorithm (RED) runsat r1, with a packet size of 1000 bytes. Note that the r1-s2 link 912 isoversubscribed by 50%, with a buffer size at r1 of 600 packets, and Ht=Lfor no hysteresis, Ht=2L for hysteresis.

Results of testing the hysteresis algorithm 301 on a User DatagramProtocol (UDP) traffic first Case 1002 on the NS-2 902 are illustratedin FIG. 9. There are 2 bursts 1004 and 1006 in the simulation. Eachburst 1004 and 1006 lasts for 2 seconds. An OFF period 1008 between theburst 1004 and 1006 is also two seconds, resulting in queue occupancyfor RED 1010 and RED-with-hysteresis 1012 as shown.

Results of testing the hysteresis algorithm 301 on a UDP short burstbursty traffic Case 1102 on the NS-2 902 are illustrated in FIG. 10. Twobursts 1104 and 1106 are run in the simulation. The first burst 1104lasts from 0 sec. to 0.5 sec. and the second burst 1106 lasts from 3.0sec. to 4.0 sec., with resultant queue occupancy for RED 1108 andRED-with-hysteresis 1110.

FIG. 11 illustrates TCP Tahoe constant traffic on the standalone networksimulator 902, wherein queue occupancy is shown for RED 1202 andRED-with-hysteresis 1204.

FIG. 12 illustrates TCP Reno bursty traffic on the standalone networksimulator 902. Two bursts 1302 and 1304 each last for one second, withan intervening OFF period 1306 of one second, wherein queue occupancy isshown for RED 1308 and RED-with-hysteresis 1310.

Lastly, FIG. 13 provides tables of packet drop and packet transmitstatistics according to the present invention.

In another embodiment of the present invention, an adaptive algorithm isalso provided to adjust the increment and decrement of transmitprobability for each flow, together with hysteresis to increase thepacket transmit rates by using packet data store to absorb burstytraffic. The proposed algorithm maintains the throughput for persistentbursts of packets. Using straight forward implementation of hysteresisalone in the active flow management will increase the system throughputby making better use of available queue capacity. However, this resultsin an increase in the queue level peak, potentially exposing the systemto tail drops when subjected to severe bursts for bursty traffic. Withthe addition of adaptive increment and decrement of transmit probabilityof each flow, queue peak can be limited to a reasonable level, thuspreventing tail drop.

An embodiment of the invention thus proposes an algorithm including thefollowing two components to improve the performance of active flowmanagement: (1) a Bandwidth Allocation Transmit (BAT) algorithm, withoutSARED but with hysteresis; and (2) an adaptive transmit fraction Tiresponsive to certain conditions (e.g., queue and/or traffic).

Further information about BAT is set forth in commonly-assigned U.S.patent applications entitled “METHOD AND SYSTEM FOR PROVIDINGDIFFERENTIATED SERVICES IN COMPUTER NETWORKS”, Ser. No. 09/448,197,filed Nov. 23, 1999, now U.S. Pat. No. 6,657,960 B1, issued to Jeffrieset al. on Dec. 2, 2003; and “METHOD AND SYSTEM FOR CONTROLLING FLOWS INSUB-PIPES OF COMPUTER NETWORKS”, Ser. No. 09/540,428, filed Mar. 31,2000, both of which are incorporated herein by this reference.

Pursuant to the U.S. Pat. No. 6,657,960 B1 and U.S. patent applicationSer. No. 09/540 428 references incorporated above, with regard to (1)the first part of the two-part algorithm (BAT without SARED but withhysteresis), the Transmit fraction of BAT for flow i, Ti, is defined asfollows:If fi(t)<fi,min then Ti(t+dt)=min(1, Ti(t)+w);else if fi(t)>fi,max then Ti(t+dt)=Ti(t)(1−w);else if B(t)=1 then Ti(t+dt)=min(1, Ti(t)+CiBavg(t));otherwise then Ti(t+dt) =Ti(t)(1−DiOi(t));where Ci and Di are constants used for increment and decrement,respectively, of Ti, fi,min is the minimum flow for the i^(th) pipe, andfi,max is the maximum flow for the i^(th) pipe. Ci and Di are defined bysubscription of each flow, fi,min, and the service rate of the system,S. They are given as follows:Ci=(S+fi,min-(fl,min+f2 ,min +. . . +fn,min))/16; andDi=(S−fi,min)*4.

Hysteresis is incorporated according to the following algorithm: ifhysteresis is on and the queue level is less than the hysteresisthreshold, then no packet will be dropped—i.e., Ti is updated but doesnot apply to packets; else, if hysteresis is off, then packets areprocessed as normal—i.e. Ti is applied to each packet.

With regard to (2) the second part of the two-part algorithm (adaptivetransmit fraction Ti based on certain conditions), prior artimplementations of BAT have been guarded by SARED which will reduce Tiwhen queue occupancy exceeds the SARED threshold, e.g., 25% of maximumqueue capacity. With hysteresis, there is no need for SARED to guardBAT. However, this may increase the queue level peak which may causetail drop due to high queue occupancy. In order to prevent packets fromtail drop, the present invention provides for an adaptive increment anddecrement of transmit probability of each flow, Ti, to prevent tail dropwhile maintaining the advantage of hysteresis, e.g. higher transmitrates with bursty traffic. An embodiment of the present inventioncomprises a normal Ti algorithm with an extended Ti algorithm to adaptTi for good conditions (low queue and/or light traffic) and severeconditions (high queue and/or severe traffic), respectively.

For conditions between good and severe, an adaptive increment anddecrement of Ti is used based on the condition of traffic or thedirection the queue level is moving. “Severe conditions” implies that noamount of congestion control will be able to prevent the discard ofarriving packets. Referring again to FIG. 1, this is the case where thequeue level (on the x axis) has reached the high threshold (H), beyondwhich the drop probability is=1. Between L and H (i.e. between the goodand severe conditions) is where the flow control lies—and with regard to(2), this is an adaptive increment and decrement of Ti unlike a fixedchange in Ti as was done in (1). The advantages of the proposedalgorithm include using queue data store to absorb short burst trafficto achieve higher throughput, and adjusting increment and decrement oftransmit probability for each flow to limit queue peak to prevent taildrop.

FIG. 14 illustrates an adaptive algorithm 1502 according to the presentinvention of Ti using hysteresis as good or severe condition indicatorfor flow i, given constants Ci and Di calculated from subscription offlow i and service rate of the system. In step 1504, the HT flag isdetermined. If HT is ON, then in step 1506 Ti is computed using Ci andDi in BAT. Else, if HT is OFF, then in step 1508 Ti is computed usingF(Ci) and G(Di) in BAT, where F is a decreasing function and G is anincreasing function. One embodiment sets F(C)=C/2 and G(D)=min(1, 2*D);however, other embodiments may utilize different values, and the presentinvention is not limited to these values. After step 1506 or step 1508,in step 1510, the Ti is updated.

HT is set according to algorithm 301 of FIG. 2. Given low threshold L,high threshold H, and hysteresis threshold Ht where 0<=L<=Ht<=H<=maximum queue capacity. Hysteresis is ON initially. It is preferredthat both Ti and HT are updated periodically and the period is dependenton queue size and service rate. When hysteresis is ON and the queuelevel is less than the hysteresis threshold, no packet will be dropped.When hysteresis is OFF, Ti is applied to every packet.

The benefit of the present invention is demonstrated by simulationresults shown in FIGS. 15 through 19. These simulations use hysteresisas an indicator of good and severe conditions. In these simulations,F(Ci)=Ci/(2^3) and G(Di)=min(1, 2^3*Di) are used. The hysteresisthreshold Ht is 2*(low threshold). The simulation contains seven casesand each has constant burst OFF duration for 1.07 ms except for the caseof 100% burst ON duration. The respective “burst ON” durations for theseven cases are: case 1602—100%; case 1604—66.6%; case 1606—61.5%; case1608—54.5%; case 1610—50.0%; case 1612—44.4%; and case 1614—37.4%. Forexample, the traffic pattern for the 66.6% burst ON duration case 1604is made by the cycles of 2.13 ms burst ON and followed by 1.07 ms burstOFF. The traffic pattern is 150% subscription when bursty traffic is ONand 20% subscription when bursty traffic is OFF.

The simulations shown in FIGS. 15 through 19 contain four (4) UDP flows,and the subscriptions of each flow are: 10%, 10%, 20%, and 0% of totalbandwidth, respectively. The offered load for each flow is 50%, 40%,20%, and 40% of total bandwidth, respectively, when burst is ON and 5%for each when burst is OFF. Note that the fourth flow is best effort—itwill only avail of the bandwidth that is left over after allocation tothe remaining flows. According to a min-max algorithm, the idealtransmit rates for each flow under constant case (100% burst ON) are60%, 75%, 100%, and 50%, respectively.

The aggregate transmit rates are illustrated in FIG. 15, and averagelatency for the same cases are shown is in FIG. 16 for various burstytraffic. FIGS. 15 and 16 show that the proposed invention can result inhigher transmit rates for bursty traffic while having higher latencybecause of the use of queue capacity to absorb bursty traffic.

Specific transmit rates for individual flows obtained through thealgorithms of the present invention 1802 and through prior art BATmethods 1804 are illustrated in FIGS. 17, 18, and 19. FIG. 17 is theaggregate transmit rate for each flow for 50.0% burst, and FIG. 18 isthe aggregate transmit rate for each flow for 66.7% burst.

FIGS. 17 and 18 show higher transmit rates for each flow for differentburstyness. They illustrate that the present invention obtains highertransmit rates for bursty traffic through better use of available queuecapacity to accommodate bursty traffic. FIG. 19 is the aggregatetransmit rate for each flow for steady traffic (100% burst). It showsthat the algorithm according to the present invention can maintain thesame transmit rate for steady, or persistent bursty, traffic.

Overall, the present invention can achieve higher aggregate transmitrates for a variety of traffic burst characteristics by making betteruse of queue capacity and can maintain the level of performance forpersistent traffic.

The algorithm for hysteresis with adaptive increment and decrement oftransmit rate can also be easily applied to Weighted RED (WRED) toachieve higher transmit rates. In this case, a different Ht thresholdcould be defined for each of the flows subscribing to the availablebandwidth along with the individual definitions of their low (Li) andhigh (Hi) thresholds. When the hysteresis flag is turned ON, theprobability of dropping can be decreased by twice of what it would bewhen the hysteresis flag is turned OFF, thereby accepting more packetsinto the queue when there is less congestion.

The invention may be tangibly embodied in a computer program residing ona computer-readable medium, such as the floppy disc 2105 or hard drive2101 shown in FIG. 20. The medium 2105 may comprise one or more of afixed and/or removable data storage device, such as a floppy disk or aCD-ROM, or it may consist of some other type of data storage. Thecomputer program may be loaded into the memory 2102 of a network managercomputer device 2110 for execution. The computer device 2110 may beconnected to a network via network interface 2103. The computer programcomprises instructions which, when read and executed by the computerdevice 2110, causes the computer device 2110 to perform the stepsnecessary to execute the steps or elements of the present invention.

The foregoing description of the exemplary embodiment of the inventionhas been presented for the purposes of illustration and description. Itis not intended to be exhaustive or to limit the invention to theprecise forms disclosed. Many modifications and variations are possiblein light of the above teaching. It is intended that the scope of theinvention be limited not with this detailed description, but rather bythe claims appended hereto.

1. A method for managing and transmitting a plurality of data packetsthrough a queue in a computer network system, comprising the steps of:determining a transmit probability of a computer network system queue asa function of an average occupancy level; determining a low levelthreshold for the queue as a fraction of a maximum capacity of the queuewherein the transmit probability is one; determining a hysteresis levelthreshold for the queue as a positive hysteresis factor multiple of thelow level threshold wherein the hysteresis level threshold is greaterthan the low level threshold and less than the maximum capacity of thequeue; initializing a hysteresis flag to ON; in response to the queuereceiving a first burst of packets wherein the hysteresis flag is set toON: (a) comparing a queue level to the hysteresis level threshold, and(b) if the queue level is less than the hysteresis level threshold, thequeue receiving and transmitting the first burst and revising thetransmit probability; or (c) else randomly dropping at least one packetfrom the first burst responsive to the transmit probability andtransmitting a remainder of the first burst packets, revising thetransmit probability and resetting the hysteresis flag to OFF; inresponse to the queue receiving a subsequent burst of packets:determining an ON/OFF state of the hysteresis flag; if the determinedhysteresis flag state is ON, then performing the hysteresis levelthreshold (a) comparing and (b) receiving, transmitting and revising or(c) dropping, transmitting, revising and resetting steps with respect tothe subsequent burst; or if the determined hysteresis flag state is OFF,then comparing a queue level to the low level threshold, and if thequeue level is less than the low level threshold and an aggregatetraffic bandwidth presented to a network link connecting the queue to anode transmitting the subsequent burst is less than a maximum bandwidthcapacity supported by the link, the queue receiving and transmitting thesubsequent burst, revising the transmit probability, and resetting thehysteresis flag to ON; or else the queue receiving the subsequent burst,randomly dropping at least one packet from the subsequent burst inresponse to the transmit probability, transmitting a remainder of thesubsequent burst packets, and revising the transmit probability.
 2. Themethod of claim 1, further comprising periodically updating the transmitprobability and the hysteresis level threshold as a function of size ofthe queue and the aggregate traffic bandwidth.
 3. The method of claim 2,wherein determining or revising the transmit probability is a functionof the hysteresis flag state.
 4. The method of claim 3, whereindetermining or revising the transmit probability comprises defining thetransmit probability as a transmit fraction Ti as a function of a dataflow parameter fi and a service rate S of the network system by applyinga bandwidth allocation transmit algorithm comprising; determining if thehysteresis flag is ON or OFF; if the flag is ON, incrementing ordecrementing Ti by:if fi(t)<=fi,min, then Ti(t+dt)=min(1, Ti(t)+w);if fi(t)>fi,max, then Ti(t+dt)=Ti(t)(1−w); if an excess bandwidth signalB(t)=1, then Ti(t+dt)=min(1, Ti(t)+CiBavg(t); orelse, Ti(t+dt)=Ti(t)(1−DiOi(t)); where Oi(t) is a current offered rateof the flow and Ci is an increment constant equal to(S+fi,min−(fl,min+f2,min+ . . . +fn,min))/16, and Di is a decrementconstant equal to (S−fi,min)*4, or if the hysteresis flag is OFF,incrementing or decrementing Ti by: if the queue level is increasing,setting Ti=F(Ci), wherein F(Ci) is a bandwidth allocation transmitdecreasing function; or if the queue level is decreasing, settingTi=G(Di), wherein G(Di) is a bandwidth allocation transmit increasingfunction.
 5. The method of claim 4, wherein F(Ci)=Ci/2 andG(Di)=min(1,2*Di).
 6. A data flow manager configured for managing andtransmitting data packets through a queue in a computer network system,comprising: a queue having a queue level in communication with the dataflow manager; and a node in communication with the queue through anetwork link, the link having a maximum bandwidth capacity; wherein thedata flow manager is configured to: determine a transmit probability ofthe queue as a function of an average queue occupancy level; determine alow level threshold for the queue as a fraction of a maximum capacity ofthe queue wherein the transmit probability is one; determine ahysteresis level threshold for the queue as a positive hysteresis factormultiple of the low level threshold wherein the hysteresis levelthreshold is greater than the low level threshold and less than themaximum capacity of the queue; in response to the queue receiving afirst burst of packets wherein a hysteresis flag is set to ON: (a)compare a queue level to the hysteresis level threshold, and (b) if thequeue level is less than the hysteresis level threshold, cause the queueto receive and transmit the first burst, the data manager furtherconfigured to revise the transmit probability; or (c) else cause thequeue to randomly drop at least one packet from the first burstresponsive to the transmit probability and transmit a remainder of thefirst burst packets, the data manager further configured to revise thetransmit probability and reset the hysteresis flag to OFF; in responseto the queue receiving a subsequent burst of packets: determine anON/OFF state of the hysteresis flag; if the determined hysteresis flagstate is ON, performing the hysteresis level threshold (a) compare and(b) receive, transmit and revise or (c) drop, transmit, revise and resetsteps with respect to the subsequent burst; or if the determinedhysteresis flag state is OFF, compare a queue level to the low levelthreshold, and: if the queue level is less than the low level thresholdand an aggregate traffic bandwidth presented to the network link is lessthan the maximum bandwidth capacity, cause the queue to receive andtransmit the subsequent burst, the data manager further configured torevise the transmit probability and reset the hysteresis flag to ON; orelse cause the queue to receive the subsequent burst, randomly drop atleast one packet from the subsequent burst in response to the transmitprobability, and transmit a remainder of the subsequent burst packets,the data manager further configured to revise the transmit probability.7. The data flow manager of claim 6 further configured to periodicallyupdate the transmit probability and the hysteresis level threshold as afunction of size of the queue and the aggregate traffic bandwidth. 8.The data flow manager of claim 7 further configured to determine orrevise the transmit probability as a function of the hysteresis flagstate.
 9. The data flow manager of claim 8 further configured todetermine or revise the transmit probability by defining the transmitprobability as a transmit fraction Ti as a function of a data flowparameter fi and a service rate S of the network system by applying abandwidth allocation transmit algorithm by: determining if thehysteresis flag is ON or OFF; if the flag is ON, incrementing ordecrementing Ti by:if fi(t)<=fi,min, then Ti(t+dt)=min(1, Ti(t)+w);if fi(t)>fi,max, then Ti(t+dt)=Ti(t)(1−w); if an excess bandwidth signalB(t)=1, then Ti(t+dt)=min(1, Ti(t)+CiBavg(t); orelse, Ti(t+dt)=Ti(t)(1−DiOi(t)); where Oi(t) is a current offered rateof the flow and Ci is an increment constant equal to(S+fi,min−(fl,min+f2,min + . . . +fn,min))/16, and Di is a decrementconstant equal to (S−fi,min)*4; or if the hysteresis flag is OFF,incrementing or decrementing Ti by: if the queue level is increasing,setting Ti=F(Ci), wherein F(Ci) is a bandwidth allocation transmitdecreasing function; or if the queue level is decreasing, settingTi=G(Di), wherein G(Di) is a bandwidth allocation transmit increasingfunction.
 10. The data flow manager of claim 9, wherein F(Ci)=Ci/2 andG(Di)=min(1,2*Di).
 11. An article of manufacture comprising a computerusable medium having a computer readable program embodied in saidmedium, wherein the computer readable program, when executed on acomputer, causes the computer to manage network data flow by:determining a transmit probability of a computer network system queue asa function of an average occupancy level; determining a low levelthreshold for the queue as a fraction of a maximum capacity of the queuewherein the transmit probability is one; determining a hysteresis levelthreshold for the queue as a positive hysteresis factor multiple of thelow level threshold wherein the hysteresis level threshold is greaterthan the low level threshold and less than the maximum capacity of thequeue; initializing a hysteresis flag to ON; in response to the queuereceiving a first burst of packets wherein the hysteresis flag is set toON: (a) comparing a queue level to the hysteresis level threshold, and(b) if the queue level is less than the hysteresis level threshold,causing the queue to receive and transmit the first burst, the computersystem revising the transmit probability; or (c) else causing the queueto randomly drop at least one packet from the first burst responsive tothe transmit probability and transmit a remainder of the first burstpackets, the computer system revising the transmit probability andresetting the hysteresis flag to OFF; in response to the queue receivinga subsequent burst of packets: determining an ON/OFF state of thehysteresis flag; if the determined hysteresis flag state is ON, thenperforming the hysteresis level threshold (a) comparing and (b)receiving, transmitting and revising or (c) dropping, transmitting,revising and resetting steps with respect to the subsequent burst; or ifthe determined hysteresis flag state is OFF, comparing a queue level tothe low level threshold, and if the queue level is less than the lowlevel threshold and an aggregate traffic bandwidth presented to anetwork link connecting the queue to a node transmitting the subsequentburst is less than a maximum bandwidth capacity supported by the link,causing the queue to receive and transmit the subsequent burst, thecomputer system revising the transmit probability and resetting thehysteresis flag to ON; or else causing the queue to receive thesubsequent burst and randomly drop at least one packet from thesubsequent burst in response to the transmit probability and transmit aremainder of the subsequent burst packets, the computer system revisingthe transmit probability.
 12. The article of manufacture of claim 11,wherein the computer readable program, when executed on a computer,further causes the computer to manage network data flow by periodicallyupdating the transmit probability and the hysteresis level threshold asa function of size of the queue and the aggregate traffic bandwidth. 13.The article of manufacture of claim 12, wherein the computer readableprogram, when executed on a computer, further causes the computer tomanage network data flow by determining or revising the transmitprobability as a function of the hysteresis flag state.
 14. The articleof manufacture of claim 13, wherein the computer readable program, whenexecuted on a computer, further causes the computer to manage networkdata flow by: determining or revising the transmit probability bydefining the transmit probability as a transmit fraction Ti as afunction of a data flow parameter fi and a service rate S of the networksystem by applying a bandwidth allocation transmit algorithm comprising;determining if the hysteresis flag is ON or OFF; if the flag is ON,incrementing or decrementing Ti by:if fi(t)<=fi,min, then Ti(t+dt)=min(1, Ti(t)+w);if fi(t)>fi,max, then Ti(t+dt)=Ti(t)(1−w); if an excess bandwidth signalB(t)=1, then Ti(t+dt)=min(1, Ti(t)+CiBavg(t); orelse, Ti(t+dt)=Ti(t)(1−DiOi(t)); where Oi(t) is a current offered rateof the flow and Ci is an increment constant equal to(S+fi,min−(fl,min+f2,min + . . . +fn,min))/16, and Di is a decrementconstant equal to (S−fi,min)*4; or if the hysteresis flag is OFF,incrementing or decrementing Ti by: if the queue level is increasing,setting Ti=F(Ci), wherein F(Ci) is a bandwidth allocation transmitdecreasing function; or if the queue level is decreasing, settingTi=G(Di), wherein G(Di) is a bandwidth allocation transmit increasingfunction.
 15. The article of manufacture of claim 14, wherein F(Ci)=Ci/2and G(Di)=min(1,2*Di).